1. Field of the Invention
This invention relates to CDMA communication systems, and more particularly to methods for initiating a reverse link intergenerational handoff in CDMA communication systems.
2. Description of Related Art
As is known, wireless communication systems facilitate two-way communication between a plurality of subscriber mobile radio stations or “mobile stations” and a fixed network infrastructure. Typically, the plurality of mobile stations communicate with the fixed network infrastructure via a plurality of fixed base stations. Exemplary systems include such mobile cellular telephone systems as Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, and Frequency Division Multiple Access (FDMA) systems. The objective of these digital wireless communication systems is to provide communication channels on demand between the mobile stations and the base stations in order to connect the mobile station users with the fixed network infrastructure (usually a wired-line system).
Exemplary CDMA Communication System
Mobile stations typically communicate with base stations using a duplexing scheme that allows for the exchange of information in both directions of connection. In most existing communication systems, transmissions from a base station to a mobile station are referred to as “forward link” transmissions. Transmissions from a mobile station to a base station are referred to as “reverse link” transmissions. These CDMA systems are well known in the art. For example, some such system is described in U.S. Pat. No. 4,901,307, issued on Feb. 13, 1990 to Gilhousen et al., which is hereby incorporated by reference for its teachings of CDMA communication systems.
Basic radio system parameters and call processing procedures for exemplary prior art CDMA systems are described in a TIA specification, entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” TIA/EIA/IS-95-A, published in May 1995 by the Telecommunications Industry Association, and referred to hereafter as “IS-95A”. The update and revision to IS-95A and J-STD-008 (PCS specification analogous to IS-95A) is TIA/EIA/IS-95-B, published in March 1999 by the TIA and referred to hereafter as “IS-95B”. The IS-95A and IS-95B specifications are jointly known as second generation or “2G” CDMA system specifications. More recently, a third generation or “3G” CDMA system has been described in the TIA specification, entitled “cdma2000 Series”, TIA/EIA/IS-2000-A, published March 2000 by the TIA, and referred to hereafter as “IS-2000”. The IS-95A, IS-95B and IS-2000 specifications are hereby incorporated by reference for their teachings on CDMA communication systems.
As shown in FIG. 1, a typical CDMA communication system comprises at least one mobile station and a plurality of fixed base stations geographically distributed over the system's service area and controlled by a mobile telecommunications switching office (MTSO) 20. The service area is defined as the geographical area within which a mobile station can remain and yet still communicate (i.e., maintain a valid radio link) with the CDMA communication system. Each base station provides communication services to a fixed area within the service area. The service area is known as the base station's “coverage area”. Thus, when a mobile station is within a base station's coverage area the base station is able to provide communication services to the mobile station. A base station that provides service to the mobile is also known as a “serving” base station. The MTSO 20 coordinates all of the switching functions between base stations, mobile stations, and other communications systems (e.g., a Public Service Telephone Network (PSTN) or satellite communication system, or the like).
As is well known, communication between a base station and a mobile station is typically established using a negotiation process initiated upon call origination. The serving base station begins the negotiation process by assigning a selected one of its available forward traffic channels to the mobile station and thus establishes a forward link with the mobile station. The mobile station then establishes a reverse link with the serving base station. Once communication is established between the serving base station and the mobile station, pilot channels emitted by each base station are used by the mobile station to determine the base station coverage area that the mobile station is within and the quality of the link to the base station. Specifically, each base station transmits an unmodulated pilot channel on a predetermined frequency that assists the mobile stations in detecting signals and measuring signal strengths of nearby base stations.
In typical CDMA systems, a mobile station maintains a list of available base stations for providing communication services to the mobile station. Normally, the mobile station communicates with a base station that has the strongest signal. The mobile station receives the pilot signals and determines which pilot signals are the strongest. A “searcher” unit in the mobile station commonly performs the signal detection and strength measurement functions.
The results from the searcher are reported to the current (i.e., active) base station. The base station then instructs the mobile station to update a list of available base stations maintained by the mobile station. The list is sub-divided into three operative sets—an active set, a candidate set, and a neighbor set. The active set contains a list of the base stations with which the mobile station is currently communicating (typically 1–4 base stations). The candidate set is a list of base stations that may move into the active set. Finally, the neighbor set is a list of base stations which are being monitored, but less frequently.
As the mobile station moves and its currently active base station signal weakens, the mobile station must access a new base station. Based upon the results of the searcher, and based upon the instructions received back from the base station, the mobile station will update its sets, and communicate with a different base station. In order for communication transmissions to appear seamless to the mobile station user, the communication link must be “handed off” to the next base station. A “handoff” occurs when a mobile station moves across a “boundary line” from a first serving base station's coverage area to a second base station's coverage area. The communication system “hands off” service from the first serving base station to the second base station, also known as the “target” base station. A handoff also occurs when a single base station utilizes multiple frequency channels and switches communication between frequency channels. Each pilot channel is identified by a pseudorandom noise (PN) sequence offset and/or a frequency assignment. Thus, each pilot channel is uniquely identified with a base station that transmits the pilot channel. Pilot channels aid mobile stations in performing handoffs.
FIG. 1 depicts a simple CDMA communication system having a service area comprising seven base stations controlled by one MTSO 20. Each base station services a separate coverage area, represented by a hexagon in FIG. 1, and communicates using a specific frequency, frequency one (F1) or frequency two (F2). Typically, F1 and F2 operate either on the Cellular band (800 MHz) or the PCS band (1900 MHz). For example, a first base station 12, located in the middle of a Service Coverage Area One, communicates on a first frequency F1. A mobile station 10 is serviced by the first base station 12 because it is located within the Coverage Area One. When the mobile station 10 moves from the Coverage Area One to a Coverage Area Two, it performs a handoff procedure from the first base station 12, the serving base station, to a second base station 14, the target base station. Thus, the mobile station 10′ (of FIG. 1) is now serviced by the second base station 14. It is critical for the MTSO to determine the appropriate time to initiate a handoff to a frequency that differs from the serving frequency in order to maintain communication with the mobile station during a call.
In CDMA systems, there are two basic types of handoffs, so-called “hard handoffs” (HHO) and “soft handoffs” (SHO). A “soft handoff” or “Make-Before-Break” is a handoff procedure in which the mobile station commences communications with a target base station without interrupting communications with the serving base station. Because mobile stations typically contain only one radio frequency (RF) chain, soft handoffs can only be used between base stations with CDMA Channels having identical frequency assignments. Referring again to FIG. 1, a soft handoff procedure can be performed when the mobile station 10 travels from a first Coverage Area One to a third Coverage Area Three because the base station 12 and a third base station 16 have identical frequency assignments, F1.
A “hard handoff” is defined as a handoff in which a mobile station commences communication with a target base station after a momentary interruption in communication with a serving base station. Hard handoffs are also referred to as “Break-Before-Make” handoffs. A hard handoff is used when the serving base stations and the target base stations have differing CDMA channel frequency assignments. A hard handoff can also occur when a single base station utilizes multiple frequency channels and switches communication between frequency channels. For example, a single base station hard handoff can occur between sectors associated with the single base station. The present invention generally addresses a multiple base station scenario, and thus, the single base station scenario is not described in detail herein. However, one skilled in the art shall recognize that the present invention can be utilized equally as well in a single base station scenario.
During a hard handoff, the radio link is momentarily interrupted because a typical mobile station contains only one RF chain and therefore can only demodulate one frequency at a time. Thus, switching from the CDMA channels of the serving base station frequency to the CDMA channels of the target base station frequency produces a momentary interruption in the continuity of the radio link with the CDMA communication system. This momentary interruption can result in a “dropped” or lost call. As shown in FIG. 1, the first base station 12 is assigned a first frequency F1 and the second base station 14 is assigned a second frequency F2. A hard handoff is performed when the mobile station 10 travels from the Coverage Area One to the Coverage Area Two because the first base station 12 and the second base station 14 operate on different frequencies, F1 and F2.
Handoffs performed within a CDMA system having base stations belonging to different generation CDMA systems (e.g., within an intergenerational CDMA system having both 2G CDMA systems and 3G CDMA systems) are known as intergenerational handoffs (IGHO). An exemplary intergenerational CDMA system and IGHO is described in more detail below with reference to FIG. 4. An IGHO can be a soft handoff or a hard handoff depending upon system factors described further below with reference to disadvantages of current methods of performing a reverse link intergenerational HHO in a CDMA communication system. 3G CDMA systems have been designed to provide backward compatibility with 2G CDMA systems at the signaling and call processing levels. However, 2G and 3G CDMA systems are not naturally compatible at the physical layer because these systems employ different modulation schemes and spreading rates. Thus, due to this physical layer incompatibility between 2G and 3G CDMA systems, problems can occur when performing IGHOs. For example, a “complete” Intergenerational Soft Handoff (IGSHO) (i.e., a soft handoff on both the forward link and the reverse link) is presently not possible.
Intergenerational CDMA systems (e.g., CDMA systems comprising both 2G and 3G CDMA systems) can perform forward link IGSHOs because mobile stations typically comprise “rake” receivers that are capable of demodulating multiple signals concurrently. Thus, a typical mobile station can simultaneously demodulate a signal from a 2G serving base station and a signal from a 3G target base station. Rake receivers and simultaneous demodulation techniques are well known in the CDMA art and thus are not described in detail herein. However, reverse link IGSHOs are not practical due to the difference in modulation and coding parameters for the different generations. Therefore, at the service boundaries between the 2G and 3G systems, a reverse link intergenerational hard handoff (IGHHO) has been proposed.
In this type of hard handoff, the connection with a currently active base station (e.g., a 2G base station) is terminated before a new service with a new base station (e.g., a 3G base station) is established. This type of service disruption lowers the quality of service (QoS) provided to the cellular telephone user. In this scenario, if the mobile station is engaged in a voice service, the user will most likely experience voice quality degradation or even a “dropped” or lost call. If the mobile station is transferring data, significant transmission delays (due to retransmission errors) will likely occur.
An important objective of CDMA communications is to reduce the probability of dropping a call during handoff procedures. As the communication system increases its accuracy of detecting the coverage boundaries between base stations, the probability of dropping a call is reduced. Thus, industry technical specifications (e.g., IS-2000) and methods have been developed to increase the accuracy of determining the best possible “handoff initiation time instant”. A handoff initiation time instant is defined herein as a moment in time when a CDMA system initiates a mobile station handoff. A mobile station performing a handoff between different generation base stations (e.g., a 2G serving base station and a 3G target base station) is susceptible to dropped calls because a reverse link IGHHO must be performed.
One method that attempts to determine the best possible handoff initiation time instant during a reverse link IGHHO utilizes pilot signal strengths (Ec/Io) and frame counters. In accordance with this method, a mobile station's searcher unit determines pilot signal strengths as described above. Pilots having Ec/Io values above a predetermined threshold (e.g., T_Add) are included in the mobile station's active set. CDMA communication systems transmit values such as T_Add using methods well known to one skilled in the CDMA communication art. When a base station is added to a mobile station's active set, the mobile station utilizes a rake receiver to simultaneously demodulate both base station signals from the 2G and 3G CDMA systems. After decoding a first “good” frame from a target base station when the target base station's Ec/Io value is above a predetermined threshold (e.g., “CT_Drop”) the mobile station initiates a frame counter. One skilled in the communication art shall recognize that a good frame is a frame having sufficient quality. The frame counter records the number of good frames received from the target base station. Upon the expiration of the frame counter a message is sent to the serving base station and the system performs a reverse link IGHHO.
FIG. 2 shows an exemplary graph depicting the above-described method of performing a reverse link IGHHO. The graph shown in FIG. 2 depicts the received pilot signal strength of a serving base station and a target base station (as measured by a mobile station) during a handoff. As shown in FIG. 2 at a time reference point A, the target base station's Ec/Io equals a pre-defined threshold “T_Add”. At T_Add, the target base station's pilot is added to the mobile station's active set and the rake receiver begins to demodulate the target base station's signal. At a time reference point B the mobile station receives a good frame from the target base station and a frame counter begins counting a predetermined number of frames. At a time reference point C the frame counter expires and a reverse link IGHHO is performed. The exemplary reverse link IGHHO of FIG. 2 is successful because the hard handoff is performed while both the serving base station and the target base station are above T_Add (the threshold for satisfactory communication between a mobile station and a base station).
Disadvantageously, the above-described method is susceptible to dropped calls because a frame counter is used to initiate the reverse link IGHHO. Use of a frame counter can cause the reverse link IGHHO to occur after the call is dropped. FIG. 3 shows one such example. FIG. 3 shows an exemplary graph depicting a disadvantage of the above-described method of performing a reverse link IGHHO. The graph shown in FIG. 3 is substantially similar to the graph shown in FIG. 2. At a time reference point A′ the target base station is added to the mobile station's active set and the rake receiver begins to demodulate the target base station's signal. At a time reference point B′ the mobile station receives a good frame from the target base station and a frame counter begins counting a predetermined number of frames. At a time reference point C′ the frame counter expires and a reverse link IGHHO is performed. As shown in FIG. 3, the Ec/Io of the serving base station fell below T_Add at the time reference point C′. Thus, the exemplary reverse link IGHHO of FIG. 3 is unsuccessful and a dropped call occurred because the mobile station lost the signal from the serving base station before initiating the reverse link IGHHO.
A disadvantage of the above-described method of performing a reverse link IGHHO in a CDMA communication system is that factors other than the signal strength (Ec/Io) affect the handoffs. For example, factors such as modulation schemes, coding gains, rate of forward link power control and coherent/non-coherent demodulation affect handoff procedures. Thus, the prior art methods of utilizing timers to determine handoff initiation time instants are susceptible to increased dropped calls or reduced voice quality. Thus, it is desirable to provide a method and apparatus for initiating mobile station handoffs between at least two different generations of CDMA systems to avoid the disadvantages associated with the prior art reverse link intergenerational hard handoff schemes. The present invention provides such a method and apparatus for initiating a reverse link intergenerational handoff in a CDMA communication system.